Role of purinergic receptors in chloride secretion in Caco-2 cells

Purinergic receptors play an important role in regulating Cl^- secretion in epithelial cells. To explore further the role of these receptors in the intestine, we utilized the human intestinal epithelial cell line, Caco-2, grown on permeable membrane supports and assayed for Cl^- secretion by measuring the short-circuit current (I(sc)). Stimulation of I(sc) by extracellular nucleotides could be detected by day 4 and increased by day 10 postseeding. The magnitude of stimulation of I(sc) at 10 μM in cells at day 10 was UTP > ATP > UDP >> 2-methylthioadenosine 5'-triphosphate (2-MeS-ATP) = ADP on the apical side and UTP = 2-MeS-ATP = ATP > ADP >> UDP on the basolateral side. Cross-desensitization studies suggested that two different receptors are expressed in the apical membrane, a P(2U) purinoceptor and a uridine nucleotide receptor. Two different receptors are also expressed in the basolateral membrane, a P(2U) receptor and another that reacts with both 2-MeS-ATP and ADP. This latter receptor has an unusual pharmacological profile, with a reactivity for 2-MeS-ATP > ADP but not for ATP. Responses to purinergic receptor agonists were inhibited by pretreatment with 1,2-bis(2-aminophenoxy)ethane- N,N,N',N'-tetraacetic acid- acetoxymethyl ester, thapsigargin, or quinine. Thus we suggest that an increase in intracellular Ca²⁺ and subsequent opening of Ca²⁺-activated K⁺ channel play a role in increasing driving force for Cl^- to exit across the apical membrane. The role of the cystic fibrosis transmembrane conductance regulator as a Cl^- exit pathway on the apical membrane was also established.